PROJECT SUMMARY/ABSTRACT The progress of Alzheimer?s disease involves interactions between multiple brain regions over years yet originates from electrophysiological changes in millisecond-scale firing events from micron-sized individual neurons. The spatiotemporal scales relevant to Alzheimer?s disease span many orders of magnitude and thus make it extremely challenging to study in the brain of live subjects. Our understanding of Alzheimer?s diseasde comes mainly from longitudinal studies with low spatiotemporal resolution (e.g., fMRI on human patients over years), and cross-sectional studies comparing different subject populations due to chronic instability (e.g., single-neuron electrophysiology with invasive brain electrodes). Neither approach can span the spatial-temporal scales necessary to resolve single-neuron activities, unravel long-range functional connections of neurons across multiple brain regions, and track the underlying neural circuit evolution at the single-unit level during the progression of Alzheimer?s disease and its consequential cognitive decline over extended time periods. This project proposes to use syringe-injectable mesh electronics, which has been demonstrated as a powerful tool for stable year-scale chronic tracking of the same individual neurons in rodent brains, for elucidating the single-neuron basis of Alzheimer?s disease. The capability of stable long-term recording, which is not possible with other brain interrogation techniques, is due to the unique tissue-like properties of mesh electronics. These properties include a flexibility comparable to brain tissue, feature sizes on the order of axons/somata, and macroporous structure that allows interpenetration and seamless integration of neural and electronic networks. Tissue-like mesh electronics produce minimal glial scarring that would otherwise insulate neurons from the neural probe and afford studies of the brain in its native state during development of Alzheimer?s disease without perturbing the endogenous distribution of neuronal and glial cells. I propose to carry out in-vivo longitudinal studies of Alzheimer?s disease in mice with single neuron resolution. In the mentored K99 phase of this award, I have accomplished design and fabrication of mesh electronic neural probes with a high multiplexity of 128 independent recording channels. I have optimized the in-vivo recording interface with high yield and small footprint, and the topology and spatial distribution of neural recording electrodes to afford more sensitive detection of single-unit action potentials from neurons in key brain regions underlying spatial memory. In addition, I have performed surgeries to implant the optimized mesh probes into hippocampus and other memory-related mouse brain regions via a minimally invasive injection process, and achieved chronic brain recordings during behavioral tests of spatial navigation. From chronic neural recording data from the same mouse brains, I have gleaned information on the age-dependent evolution of neural circuit connectivity, providing insight on the single-neuron basis of memory retention deficit and learning impairment during brain aging. In the independent R00 phase of this award, I plan to focus on correlation of chronic in-vivo brain recordings acquired through a wireless interface with memory-related behavioral task performance, from which causal links will be derived between chronically probed neural connectivity/plasticity and the animals? behavioral performance during the progression of Alzheimer?s disease in a transgenic mouse model. Moreover, I will also focus on further development of this technology through localized electrical and optical stimulation with simultaneous electrophysiological recording of neural activity, to explore potential strategies for ameliorating deleterious changes in brain circuitry associated with memory loss and cognitive decline due to Alzheimer?s. The proposed research projects will demonstrate tissue-like mesh electronics, along with other neurotechnologies to be developed in my independent research lab for minimally-invasive brain interrogation and modulation, as transformative tools for addressing the real-world medical challenges of Alzheimer?s disease and cognitive aging in the brain.